lactose molecule

Lactose molecule.

Lactose is a disaccharide (12-carbon) sugar that occurs in mammalian milk. It consists of a molecule of glucose and one of galactose joined by a glycosidic bond and is broken down into these simple sugars by the action of the enzyme lactase which is secreted in the small intestine. Human milk contains about 6.7% lactose; cows' milk about 4.5% lactose. It exists in milk at body temperature mainly in two tautomeric forms (α and β) in approximately the proportion 2 to 3.


People with lactase deficiency have a reduced ability to digest lactose – a condition known as lactose intolerance. In rare cases, lactose intolerance may occur in someone who is not deficient in lactase.


Lactosuria is the presence of lactose in the urine. This often occurs during pregnancy and breastfeeding or if the milk flow is suppressed.


Lactose is important in cheese-making, when lactic bacteria turn it into lactic acid, so souring the milk in the production of cheese curd.